1. Field of the Invention
This invention relates to a method of encapsulating chemical biological materials in a matrix of water-insoluble starch-based adducts, and to the compositions prepared thereby.
2. Description of the Prior Art
Prior art methods of encapsulation can be described in two major categories, physicomechanical and chemical. Physicomechanical techniques include the following: spray drying: dipping or centrifuging technique; multiple nozzle spraying; fluidized bed coating; electrostatic microencapsulation; and vacuum encapsulation. The most important chemical encapsulation techniques include simple and complex coacervation and interfacial polymerization, the latter of which encompasses the prior art most relevant to the instant invention. The interfacial polymerization method necessitates the use of at least a two-phase system. One of the reactants must be soluble in the continuous phase and insoluble in the discontinuous phase (core material). The other reactant must be insoluble in the continuous phase and soluble in the discontinuous phase. The polymerization reaction occurs at the interface between the two phases forming a polymer shell around the core material, thereby completely enveloping it. This shell must be insoluble in both phases. In this method either phase can be an aqueous system. See U.S. Pat. Nos. 3,577,515 and 3,575,882 and British Pat. No. 1,163,023.
The above-mentioned encapsulation methods are multistep processes which require carefully controlled conditions or special equipment. They are time consuming and expensive, often requiring elevated temperatures and pressures other than ambient; and they all require at least a two-phase system. Many require expensive, toxic, and flammable solvents which must be recovered. Coacervation is limited to the encapsulation of oils in materials which have the capacity to form gels. Interfacial polymerization techniques, also requiring two or more phases, are limited essentially to expensive synthetic polymerization systems, many of which are petrochemicals and which generally produce nonbiodegradable polymers. To make these systems more economical and to prevent ecological contamination, unreacted monomers must be recovered. The only system that appears to be useful for coating solid particles is the fluidized bed technique.
An alternative to the conventional interfacial polymerization methods is the film-forming system as taught in Connick [Proc. of the 6th International Symp. on Controlled Release of Bioactive Materials, pp. III-1 to III-3 (Aug. 6-8, 1979)]. In accordance with this method, the active ingredient dispersed in an aqueous solution of about 1% sodium alginate is added dropwise to a gellant bath consisting of a metal salt such as calcium chloride to form small gel spheres. While this procedure is applicable to the encapsulation of a wider variety of chemical substances and formulations than interfacial polymerization, it is inherently slow, it does not lend itself to the incorporation of high concentrations of active ingredient, and the product is not easily recovered from the gellant bath.
In copending applications Ser. No. 733,968, filed Oct. 19, 1976, now U.S. Pat. No. 4,277,364, and Ser. No. 150,550, filed May 16, 1980, now U.S. Pat. No. 4,344,857, disclose methods of encapsulation whereby a polyhydroxy polymer xanthate and a coupling agent are reacted from a single phase to form an insolubilized matrix, thereby entrapping the chemical biological agent. The Shasha et al. system has the advantage of operating in both aqueous and nonaqueous two-phase systems, as well as in single-phase systems in which the matrix-forming materials and the core material are soluble in the same solvent. Another advantage is that the entire reaction mixture is converted into a solidified mass which is readily dried and ground into a usable product after pressing out excess water. This obviates the recovery from a liquid medium as necesssitated by most coacervation and interfacial polymerization methods. However, widespread commercial acceptance of the Shasha et al. methods has been hindered by the reluctance of the industry to handle carbon disulfide which is flammable and toxic.